1. Field of the Invention
The present invention relates to a power supply device for supplying electric power from a power source to a load.
2. Description of the Prior Art
A conventional example of the circuit configuration of a power supply device designed for use in car-mounted audio equipment or the like is shown in FIG. 4. As shown in this figure, in this circuit, to obtain high current capacity, and to prevent backflow current from a smoothing capacitor C to a battery, the supply voltage VCC from the battery is fed out through a voltage follower 101 composed of two NPN-type transistors 111 and 112 that are connected to form a Darlington pair and a diode D connected in a forward direction between the collector of the transistor 112 and the supply voltage VCC. A resistor 114 is connected to the base of the transistor 111.
A constant voltage generating circuit 102 operates on the voltage VCCxe2x80x2 output from the voltage follower 101 to produce a constant voltage. The voltage VOUT output from the constant voltage generating circuit 102 is fed, for example, to a microcomputer provided in car-mounted audio or video equipment (not shown) as its operating voltage.
In a car, when the engine is started, or when the audio equipment or the like is switched on, a sharp, instantaneous drop is very likely in the supply voltage supplied from the car battery. On the other hand, on the part of a load, an instantaneous drop in the operating voltage supplied thereto that goes beyond the tolerable range may cause, for example, unintended resetting of a microcomputer. Hereinafter, intended operation will be referred to as xe2x80x9cnormal operation.xe2x80x9d
The conventional power supply device configured as described above produces an output voltage that is lower than the supply voltage by as much as nxc3x97VF, where n represents the number of transistors constituting the voltage follower 101 and VF represents the voltage drop between the base and emitter of each transistor. Therefore, in the event of an instantaneous drop in the supply voltage as mentioned above, this power supply device causes a sharper drop in the operating voltage supplied to a load. Thus, this power supply device demands that the supply voltage be kept within a comparatively narrow range to ensure normal operation of the load. From a different perspective, this requires that a load be designed specially to operate normally on a lower minimum operating voltage, for example by designing its microcomputer to be reset at a lower voltage, and thus leads to higher costs.
An object of the present invention is to provide a power supply device that tolerates a wider range for a drop in a supply voltage to ensure normal operation of a load.
To achieve the above object, according to one aspect of the present invention, a power supply device that produces a predetermined voltage from a supply voltage through a voltage follower composed of a plurality of transistors is provided with: a switch circuit connected between the supply voltage and the control electrode of one of the transistors constituting the voltage follower other than the first-stage transistor thereof; and a voltage drop detecting circuit for turning on the switch circuit when a level of the supply voltage is lower than a threshold level.
In this circuit configuration, if it is assumed that the switch circuit is connected to the control electrode of the k-th stage transistor, as counted from the input side, among the transistors that are Darlington-connected to form the voltage follower, the difference in voltage by which the voltage on the output side of the voltage follower is lower than the supply voltage is nxc3x97VF (where n represents the number of transistors that are Darlington-connected to form the voltage follower) when the switch circuit is off, but, when the switch circuit is turned on, this difference in voltage reduces to (nxe2x88x92k+1)xc3x97VF equivalent to the sum of the voltage drops across part of the transistors and across the switch circuit itself. Thus, by appropriately setting the threshold level, it is possible to secure high current capacity as long as no drop is detected in the voltage supplied from the power supply device to a load, and, even if a drop in the supply voltage causes a drop in the voltage supplied from the power supply device to the load, it is possible to minimize the drop.
According to another aspect of the present invention, a power supply device that produces a predetermined voltage from a supply voltage through a voltage follower composed of a transistor is provided with: a switch circuit connected between the supply voltage and the output side of the voltage follower; and a voltage drop detecting circuit for turning on the switch circuit when a level of the supply voltage is lower than a threshold level.
In this circuit configuration, the difference in voltage by which the voltage on the output side of the voltage follower is lower than the supply voltage is nxc3x97VF when the switch circuit is off, but, when the switch circuit is turned on, this difference in voltage reduces to a value equivalent to the voltage drop across the switch circuit alone. Thus, by appropriately setting the threshold level, it is possible to secure high current capacity as long as no drop is detected in the voltage supplied from the power supply device to a load, and, even if a drop in the supply voltage causes a drop in the voltage supplied from the power supply device to the load, it is possible to minimize the drop.
As the switch circuit mentioned above, it is possible to use, for example, the output-side transistor of a current mirror circuit. The threshold voltage mentioned above is, for example in cases where a constant voltage generating circuit is additionally provided that operates on the output voltage from the voltage follower to produce a constant voltage, set equal to the level of the supply voltage at which the output voltage from the constant voltage generating circuit starts falling.